It is possible that there may be a Bing Search update happening now - although Bing updates are always much harder to track.
One webmaster said:
Anyone else notice any huge fluctuations in the Bing index over the holiday weekend. Might be a bug, but seems that Bing dropped nearly their entire index for non-signed in/connected searches.
This is affecting all sites, local and national. For example, Amazon.com has 52 million pages indexed when signed into Bing with my FB account and 42, not 42 million, 42 total, when signed out. Same goes for Target, 1.2 million signed in, 40 signed out.
Noticed this on a variety of client sites based on wide fluctuations in rankings and tested across a variety of verticals and is the same. At the local level, it's even more drastic with many of the sites I tested completely de-indexed when signed out.
Granted, I know the index can vary, but something major seems to be happening, whether an update or a bug...and site rankings back this up across a variety of client and competitor sites that we track.
Another webmaster confirmed this person's findings but again, since webmasters are less likely to track Bing's referrer traffic, it is hard to confirm if there was an update.
Did you notice changes at Bing?
Last time we thought there may have been a Bing update was in June.